This invention relates to mechanisms for retaining or locking a ball nut-screw mechanism when the actuating motor is de-energized. The mechanism would find principle use in a situation where the motor is connected to the screw to rotate it in the forward direction, and a heavy load is connected to the nut to produce a potentially dangerous reverse rotation of the screw when the motor is de-energized. At the moment of motor-deenergization the load can exert a tremedous axial force on the nut; the very low frictional connection defined by the circulating balls tends to produce a substantial reverse torque on the screw so that the screw and connected rotor (armature) can reverse rotate at tremedous speeds sufficient to destruct the motor.
An aim of the present invention is to provide a brake mechanism that automatically locks the nut and screw together when the electric motor actuator is de-enerized. The brake is designed so that it exerts a braking action only when the motor is de-energized, not when the motor is running. Therefore the low friction advantages of the ball nut-screw structure are retained without having the dangerous high speed reverse rotation of the screw.
The mechanism of this invention is somewhat similar to the brake mechanism shown in U.S. Pat. No. 3,924,479 issued to H. Lanzenberger on Dec. 9, 1975, except that in my mechanism the brake action takes place between the nut and screw. Lanzenberger apparently provides a brake between the screw and stationary support surface. One disadvantage in the Lanzenberger arrangement is that the brake is continually engaged so that the actuating motor must overcome the brake friction in order to rotate the screw; a relatively large motor is required.